Assuming:
change1 = _mm_movemask_epi8(bytemask);
offset = popcnt(change1);
On large buffers, using two shuffles and a 1 KiB table is only ~10% slower than using 1 shuffle and a 1MiB table. My attempts at generating the shuffle mask via prefix sums and bit twiddling are about about half the speed of the table based methods
(solutions using pext/pdep were not explored).
Reducing table size: Use two lookups into a 2 KiB table instead of 1 lookup into a 1 MiB table. Always keep the top-most byte - if that byte is to be discarded then it doesn't matter what byte is at that position (down to 7-bit indices, or 1 KiB table). Further reduce possible combinations by manually packing the two bytes in each 16-bit lane (down to a 216 byte table).
The following example strips whitespace from text using SSE4.1. If only SSSE3 is available then blendv can be emulated. The 64-bit halves are re-combined by overlapping writes to memory, but they could be re-combined in the xmm register (as seen in the AVX2 example).
#include <stdint.h>
#include <smmintrin.h> // SSE4.1
size_t despacer (void* dst_void, void* src_void, size_t length)
{
uint8_t* src = (uint8_t*)src_void;
uint8_t* dst = (uint8_t*)dst_void;
if (length >= 16) {
// table of control characters (space, tab, newline, carriage return)
const __m128i lut_cntrl = _mm_setr_epi8(' ', 0, 0, 0, 0, 0, 0, 0, 0, '\t', '\n', 0, 0, '\r', 0, 0);
// bits[4:0] = index -> ((trit_d * 0) + (trit_c * 9) + (trit_b * 3) + (trit_a * 1))
// bits[15:7] = popcnt
const __m128i sadmask = _mm_set1_epi64x(0x8080898983838181);
// adding 8 to each shuffle index is cheaper than extracting the high qword
const __m128i offset = _mm_cvtsi64_si128(0x0808080808080808);
// shuffle control indices
static const uint64_t table[27] = {
0x0000000000000706, 0x0000000000070600, 0x0000000007060100, 0x0000000000070602,
0x0000000007060200, 0x0000000706020100, 0x0000000007060302, 0x0000000706030200,
0x0000070603020100, 0x0000000000070604, 0x0000000007060400, 0x0000000706040100,
0x0000000007060402, 0x0000000706040200, 0x0000070604020100, 0x0000000706040302,
0x0000070604030200, 0x0007060403020100, 0x0000000007060504, 0x0000000706050400,
0x0000070605040100, 0x0000000706050402, 0x0000070605040200, 0x0007060504020100,
0x0000070605040302, 0x0007060504030200, 0x0706050403020100
};
const uint8_t* end = &src[length & ~15];
do {
__m128i v = _mm_loadu_si128((__m128i*)src);
src += 16;
// detect spaces
__m128i mask = _mm_cmpeq_epi8(_mm_shuffle_epi8(lut_cntrl, v), v);
// shift w/blend: each word now only has 3 states instead of 4
// which reduces the possiblities per qword from 128 to 27
v = _mm_blendv_epi8(v, _mm_srli_epi16(v, 8), mask);
// extract bitfields describing each qword: index, popcnt
__m128i desc = _mm_sad_epu8(_mm_and_si128(mask, sadmask), sadmask);
size_t lo_desc = (size_t)_mm_cvtsi128_si32(desc);
size_t hi_desc = (size_t)_mm_extract_epi16(desc, 4);
// load shuffle control indices from pre-computed table
__m128i lo_shuf = _mm_loadl_epi64((__m128i*)&table[lo_desc & 0x1F]);
__m128i hi_shuf = _mm_or_si128(_mm_loadl_epi64((__m128i*)&table[hi_desc & 0x1F]), offset);
// store an entire qword then advance the pointer by how ever
// many of those bytes are actually wanted. Any trailing
// garbage will be overwritten by the next store.
// note: little endian byte memory order
_mm_storel_epi64((__m128i*)dst, _mm_shuffle_epi8(v, lo_shuf));
dst += (lo_desc >> 7);
_mm_storel_epi64((__m128i*)dst, _mm_shuffle_epi8(v, hi_shuf));
dst += (hi_desc >> 7);
} while (src != end);
}
// tail loop
length &= 15;
if (length != 0) {
const uint64_t bitmap = 0xFFFFFFFEFFFFC1FF;
do {
uint64_t c = *src++;
*dst = (uint8_t)c;
dst += ((bitmap >> c) & 1) | ((c + 0xC0) >> 8);
} while (--length);
}
// return pointer to the location after the last element in dst
return (size_t)(dst - ((uint8_t*)dst_void));
}
Whether the tail loop should be vectorized or use cmov is left as an exercise for the reader. Writing each byte unconditionally/branchlessly is fast when the input is unpredictable.
Using AVX2 to generate the shuffle control mask using an in-register table is only slightly slower than using large precomputed tables.
#include <stdint.h>
#include <immintrin.h>
// probably needs improvment...
size_t despace_avx2_vpermd(const char* src_void, char* dst_void, size_t length)
{
uint8_t* src = (uint8_t*)src_void;
uint8_t* dst = (uint8_t*)dst_void;
const __m256i lut_cntrl2 = _mm256_broadcastsi128_si256(_mm_setr_epi8(' ', 0, 0, 0, 0, 0, 0, 0, 0, '\t', '\n', 0, 0, '\r', 0, 0));
const __m256i permutation_mask = _mm256_set1_epi64x( 0x0020100884828180 );
const __m256i invert_mask = _mm256_set1_epi64x( 0x0020100880808080 );
const __m256i zero = _mm256_setzero_si256();
const __m256i fixup = _mm256_set_epi32(
0x08080808, 0x0F0F0F0F, 0x00000000, 0x07070707,
0x08080808, 0x0F0F0F0F, 0x00000000, 0x07070707
);
const __m256i lut = _mm256_set_epi32(
0x04050607, // 0x03020100', 0x000000'07
0x04050704, // 0x030200'00, 0x0000'0704
0x04060705, // 0x030100'00, 0x0000'0705
0x04070504, // 0x0300'0000, 0x00'070504
0x05060706, // 0x020100'00, 0x0000'0706
0x05070604, // 0x0200'0000, 0x00'070604
0x06070605, // 0x0100'0000, 0x00'070605
0x07060504 // 0x00'000000, 0x'07060504
);
// hi bits are ignored by pshufb, used to reject movement of low qword bytes
const __m256i shuffle_a = _mm256_set_epi8(
0x7F, 0x7E, 0x7D, 0x7C, 0x7B, 0x7A, 0x79, 0x78, 0x07, 0x16, 0x25, 0x34, 0x43, 0x52, 0x61, 0x70,
0x7F, 0x7E, 0x7D, 0x7C, 0x7B, 0x7A, 0x79, 0x78, 0x07, 0x16, 0x25, 0x34, 0x43, 0x52, 0x61, 0x70
);
// broadcast 0x08 then blendd...
const __m256i shuffle_b = _mm256_set_epi32(
0x08080808, 0x08080808, 0x00000000, 0x00000000,
0x08080808, 0x08080808, 0x00000000, 0x00000000
);
for( uint8_t* end = &src[(length & ~31)]; src != end; src += 32){
__m256i r0,r1,r2,r3,r4;
unsigned int s0,s1;
r0 = _mm256_loadu_si256((__m256i *)src); // asrc
// detect spaces
r1 = _mm256_cmpeq_epi8(_mm256_shuffle_epi8(lut_cntrl2, r0), r0);
r2 = _mm256_sad_epu8(zero, r1);
s0 = (unsigned)_mm256_movemask_epi8(r1);
r1 = _mm256_andnot_si256(r1, permutation_mask);
r1 = _mm256_sad_epu8(r1, invert_mask); // index_bitmap[0:5], low32_spaces_count[7:15]
r2 = _mm256_shuffle_epi8(r2, zero);
r2 = _mm256_sub_epi8(shuffle_a, r2); // add space cnt of low qword
s0 = ~s0;
r3 = _mm256_slli_epi64(r1, 29); // move top part of index_bitmap to high dword
r4 = _mm256_srli_epi64(r1, 7); // number of spaces in low dword
r4 = _mm256_shuffle_epi8(r4, shuffle_b);
r1 = _mm256_or_si256(r1, r3);
r1 = _mm256_permutevar8x32_epi32(lut, r1);
s1 = _mm_popcnt_u32(s0);
r4 = _mm256_add_epi8(r4, shuffle_a);
s0 = s0 & 0xFFFF; // isolate low oword
r2 = _mm256_shuffle_epi8(r4, r2);
s0 = _mm_popcnt_u32(s0);
r2 = _mm256_max_epu8(r2, r4); // pin low qword bytes
r1 = _mm256_xor_si256(r1, fixup);
r1 = _mm256_shuffle_epi8(r1, r2); // complete shuffle mask
r0 = _mm256_shuffle_epi8(r0, r1); // despace!
_mm_storeu_si128((__m128i*)dst, _mm256_castsi256_si128(r0));
_mm_storeu_si128((__m128i*)&dst[s0], _mm256_extracti128_si256(r0,1));
dst += s1;
}
// tail loop
length &= 31;
if (length != 0) {
const uint64_t bitmap = 0xFFFFFFFEFFFFC1FF;
do {
uint64_t c = *src++;
*dst = (uint8_t)c;
dst += ((bitmap >> c) & 1) | ((c + 0xC0) >> 8);
} while (--length);
}
return (size_t)(dst - ((uint8_t*)dst_void));
}
For posterity, the 1 KiB version (generating the table is left as an exercise for the reader).
static const uint64_t table[128] __attribute__((aligned(64))) = {
0x0706050403020100, 0x0007060504030201, ..., 0x0605040302010700, 0x0605040302010007
};
const __m128i mask_01 = _mm_set1_epi8( 0x01 );
__m128i vector0 = _mm_loadu_si128((__m128i*)src);
__m128i vector1 = _mm_shuffle_epi32( vector0, 0x0E );
__m128i bytemask0 = _mm_cmpeq_epi8( ???, vector0); // detect bytes to omit
uint32_t bitmask0 = _mm_movemask_epi8(bytemask0) & 0x7F7F;
__m128i hsum = _mm_sad_epu8(_mm_add_epi8(bytemask0, mask_01), _mm_setzero_si128());
vector0 = _mm_shuffle_epi8(vector0, _mm_loadl_epi64((__m128i*) &table[(uint8_t)bitmask0]));
_mm_storel_epi64((__m128i*)dst, vector0);
dst += (uint32_t)_mm_cvtsi128_si32(hsum);
vector1 = _mm_shuffle_epi8(vector1, _mm_loadl_epi64((__m128i*) &table[bitmask0 >> 8]));
_mm_storel_epi64((__m128i*)dst, vector1);
dst += (uint32_t)_mm_cvtsi128_si32(_mm_unpackhi_epi64(hsum, hsum));
https://github.com/InstLatx64/AVX512_VPCOMPRESSB_Emu has some benchmarks.
